This invention relates to infrared detectors and, more particularly, to thermopile infrared detectors.
Detectors of the kind to which the present invention refers are based on a phenomenon known as the Seebeck effect. This effect occurs when two conductors of different materials are joined by junctions at their ends and one junction is maintained at a higher temperature than the other, causing a voltage difference to arise, and an electric current to flow between the hot and the cold junctions. Since for a given combination of martials the voltage difference varies in direct proportion to the temperature difference, the Seebeck effect has been utilized for the accurate measurement of temperature by means of a thermocouple detector in which one junction is maintained at a known reference temperature and the other at the location where the temperature is unknown. By measuring the voltage drop across the junctions, the unknown temperature can be deduced. Due to their very low electrical impedance, thermocouples require the use of a low-input impedance readout circuit, although such a circuit produces excessive electrical noise. To avoid this problem, it has been known to increase the impedance of thermocouple detectors by electrically connecting several thermocouples in series forming a thermopile detector.
In known infrared imaging applications, a plurality of thermopiles is connected in series to form a pixel, and many pixels are packed in an array with the geometry of the thermopiles and the spacing between them being small enough to provide acceptable image resolution and a fast response time.
A number of patents disclose various thermopile arrays and their method of production using semiconductor micro-processing technology, which substantially reduces the cost of mass production. U.S. Pat. No. 4,558,342, U.S. Pat. No. 5,059,543, and U.S. Pat. No. 6,046.398 disclose the formation of thin thermocouples, which lie flat on the surface of a thin supporting dielectric membrane and are generally perpendicular to the direction of incident radiation to be detected.
The present invention provides for a thermoelectric infrared detector to be exposed to incident infrared radiation comprising a substrate And two kinds of conducting pillars longitudinally extending away from the substrate towards the incident radiation. The pillars have upper, hot ends that are remote from the substrate and lower ends at the substrate. Pairs of adjacent pillars of different kinds are electrically connected by conducting junctions at their upper ends, and thereby define a plurality of thermocouples, with the junctions being exposed to incident infrared radiation. The thermocouples may be connected in series to form pixels, which may then be grouped into a detector array.
With the configuration of thermocouples in accordance with the present invention, the substrate and pillars are hidden from incident radiation by the junctions, which occupy most of the area exposed thereto, thereby allowing for a more sensitive and more efficient detection. In order to further maximize the area from which the junctions may benefit, the junctions may also be covered by a thin, electrically insulated membrane, which will capture radiation falling in the spaces between the junctions and conduct it thereto.
The sensitivity of the detector of the present invention to incident radiation can be estimated as follows. For a semiconducting pillar of height l and square cross-section d, the thermal conductance, Gc, of the pillar is:
Gc≈xcexxc2x7d2/l,xe2x80x83xe2x80x83(1)
where xcex is a specific heat conductivity.
The heat conductance due to thermal radiation can be expressed by:
Gr=8xc2x7xcfx80xc2x7xcex7xc2x7"sgr"xc2x7dxc2x7T3(d+4xc2x7l),xe2x80x83xe2x80x83(2)
where xcex7 is emissivity and "sgr" is Stefan""s constant.
The total thermal conductance Gt is the sum of Gc and Gr.
Heat capacitance, H, of the pillar is given by:
H=Cxc2x7d2xc2x7lxe2x80x83xe2x80x83(3)
and the thermal time constant is given by:
xcfx84=H/Gtxe2x80x83xe2x80x83(4)
Equation 4 sets the dependence between l and d for every required time constant, xcfx84. Calculations show that, for example, for d=1-50 xcexcm and xcfx84=1-10 msec, the length of the pillar should be made l=50-350 xcexcm.
Noise in the detector""s output signal originates mainly from thermal fluctuations in the detector material and from the Johnson""s noise incurred by the pillar""s electrical resistance R, which is given by:
R=xcfx81xc2x7l/d2
where xcfx81 is specific resistance of the semiconductor.
Noise equivalent power (NEP) of the thermopile, which is the inverse of detectivity, can be estimated as:   NEP  =                                          4            ·            k            ·            T            ·                          G              I                                            d            2                          ⁢                  xe2x80x83                ⁢                  (                      T            +                                          R                ·                                  G                  I                                                                              η                  2                                ·                                  P                  2                                                              )                      ·          f      .      
where k is Boltzmann""s constant, f is filling factor and P is Seebeck coefficient. Since the Seebeck coefficient of a semiconductor increases with increasing conductivity, it is always preferable to employ the most conductive, but not degenerate, semiconductor. For example, if the filling factor is about 50%, xcex7≈1, xcfx81=0.5 xcexa9xc2x7cm and P≈2 mV/K, then the NEP is in the 10xe2x88x928 Wsxc2xd/cm range for a silicon based detector. This is just few a hundred times below the ultimate theoretical limit set by thermal fluctuations (8xc2x710xe2x88x9211 Wsxc2xd/cm at 25xc2x0 C.).
FIG. 11 shows a graph of the NEP, measured in Wsxc2xd/cm, for two different time responses of a silicon based detector of the present invention as a function of pillar thickness d, measured in xcexcm. Here, the Seebeck coefficient P is 3 mV/K and the specific resistance xcfx81 is 0.1 xcexa9xc2x7cm. The solid line represents a time response xcfx84 of 10 ms, which corresponds to a pillar height l of 350 xcexcm. The dashed line represents a time response xcfx84 of 1 ms, corresponding to a pillar height l of 150 xcexcm.
The calculations given above set the lower border of the performance of a detector of the present invention, because it was assumed that the pillars are evenly heated. In practice, it may be enough to warm a part of the pillar at its upper end, to a depth equal to the charge carrier mean free path in order to get the same value of thermoelectric power. Therefore, the NEP will actually be 5 to 50 times larger depending on the pillar height and material. Calculations indicate that a detector manufactured from a semiconductor having a larger carrier mobility and a lower thermal conductivity than that of silicon, such as GaAs for example, will operate at the thermodynamic detectivity limit thereby improving performance.
The performance of the detector can be further improved by using mushroom-shaped pillars, thereby increasing the radiation absorption area of the pillars without increasing their width. This can also be achieved by covering the pillar by a thin, electrically-insulated heat-conducting continuous or non-continuous membrane.
The detector of the present invention may be manufactured by the use of known integrated technology methods. Thus, for example, the pillars can be fabricated by subsequent doping and Deep Reactive Ion Etching (DRIE) of a semiconductor substrate. The pillars can also be fabricated from a wide variety of thermoelectric materials and by different means, including additive technology.
The manufacturing process for a detector according to the present invention is compatible with most of the currently used micro-fabrication practices and, therefore, may be considerably less expensive.